Leaders of the 27 other European Union countries will meet in Brussels without the UK to thrash out their plans for handling the Brexit talks.
The discussion, which will take place after Theresa May has left a summit meeting of all EU leaders, is expected to see the European Commission's top Brexit official Michel Barnier given the lead role in the negotiations.
British sources welcomed the talks, saying they showed that the EU recognised that the UK would be leaving the bloc as promised by the Prime Minister.
The discussion will decide how the EU will organise itself for the talks which will follow Mrs May triggering Article 50, the formal legal mechanism which officially begins the two-year countdown to Brexit.
"It shows that they are facing up to the reality that the UK is leaving the EU, that we are going to be triggering Article 50 by the end of March, that means they are going to need to know how they are going to handle the process where they have got to work out the position of 27," a Number 10 source said.
The 27 national leaders are expected to agree the plan over a working dinner in the Belgian capital.
But there could be complications as MEPs claimed the leaders' plan - revealed in a leaked draft - sidelined the European Parliament's Brexit negotiator Guy Verhofstadt.
The former Belgian prime minister said: "If the government leaders do not take the European Parliament's role seriously, we'll negotiate directly with the British. If that's what they want, they'll get it."
Giving officials from president Jean-Claude Juncker's European Commission the central role could also weaken the ability of Mrs May and her ministers to use their influence directly with counterparts in other governments to secure a favourable Brexit deal.
Mrs May will hold talks with the outgoing president of the European Parliament Martin Schulz in the margins of the Brussels summit.
She will also meet Latvian prime minister Maris Kucinskis and Lithuanian president Dalia Grybauskaite, meaning that she will have held talks with leaders from all EU countries apart from Austria and Bulgaria by the time the summit starts.